The present invention relates to a process for preparing a carbonic dialkyl ester or dialkyl carbonate (to be referred to as carbonic diester) such as dimethyl carbonate by reaction of an alkyl nitrite such as methyl nitrite and carbon monoxide by vapor phase catalytic in the presence of a solid catalyst, wherein platinum group metal ion is exchanged on an ion exchange zeolite carrier.
Carbonic dialkylesters such as dimethyl carbonate are extremely useful as raw materials and for synthesis of aromatic polycarbonates and various chemical products, and as solvents.
For example, a process is proposed in Japanese Unexamined Patent Publication No. 181051/1985 for the preparation of dimethyl carbonate by reacting methyl nitrite and carbon monoxide in the vapor phase in the presence of a solid catalyst, wherein a platinum group metal or its compound is carried on a carrier such as activated carbon, and oxidant at 10 mol % or more as oxygen per carbon monoxide.
However, in this process, despite an oxidant such as oxygen being present in the above-mentioned proportion with respect to carbon monoxide in order to suppress production of dimethyl oxalate byproduct, a considerable amount of dimethyl oxalate is produced at high selectivity. This results in the problems of decreased selectivity of dimethyl carbonate, reduced reaction rate and low space time yield (STY to be described later). In addition, when reacting the mixed gas consisting of methyl nitrite, carbon monoxide, oxygen and so forth, there is the risk of explosion thus resulting in additional problems in terms of safety during the reaction.
More recently, a process for preparing dimethyl carbonate by vapor phase Catalytic reaction of carbon monoxide and methyl nitrite is disclosed in Japanese Unexamined Patent Publication No. 141243/1991 (Japanese Patent Application No. 274816/1989. In this process, a solid catalyst is used wherein a halogen compound such as a chloride of a platinum group metal, and a halogen compound such as the chloride of a secondary component metal selected from the group consisting of iron, copper, cobalt, nickel and tin, are carried on activated carbon and so on.
Moreover, Japanese Unexamined Patent Publication No. 89458/1992 (Japanese Patent Application No. 201146/1990) discloses a modified process wherein a minute amount of hydrogen chloride is added to the reaction system in the above-mentioned preparing of dimethyl carbonate.
However, in the above-mentioned preparing processes, the activity of the catalyst gradually decreases due to the releasing (dispersing) of minute amounts of chlorine from the catalyst. Thus, in order to prevent this, it is necessary to supply a minute amount of hydrogen chloride to the reaction system. As a result, said processes have the problems of chlorine ending up contaminating the target product of dimethyl carbonate, and the potential for the reaction apparatus being corroded by the chlorine.